EP1534942A1

EP1534942A1 - Controlled temperature combustion engine

Info

Publication number: EP1534942A1
Application number: EP03721386A
Authority: EP
Inventors: Charles L. Gray, Jr.
Original assignee: US Environmental Protection Agency
Current assignee: US Environmental Protection Agency
Priority date: 2002-08-08
Filing date: 2003-03-19
Publication date: 2005-06-01
Anticipated expiration: 2023-03-19
Also published as: AU2003224703B2; CN1675456A; CA2491390A1; WO2004015255A1; EP1534942B1; US6651432B1; JP2008185041A; KR20050034718A; CA2491390C; KR101007495B1; JP4364122B2; CN100467839C; EP1534942A4; AU2003224703A1; JP2005535823A

Abstract

A method of operating an internal combustion engine (22), wherein intake ambient air is boosted to a higher pressure by passage through at least one compressor (19) and then introduced into the internal combustion engine (22). Fuel is also introduced into the internal combustion engine (22) for providing combustion in admixture with the air charge at a combustion temperature approximating a target value. Various engine operating parameters, inclusive of torque demand, e.g., accelerator pedal depression (33), are sensed and the boosted pressure is changed in a manner proportional to a change in the sensed torque demand so as to maintain the combustion temperature at approximately the target value, i.e., below 2100°K.

Description

CONTROLLED TEMPERATURE COMBUSTION ENGINE

BACKGROUND OF THE INVENTION Field of the Invention

[0001] The invention is directed to a reduction of NO_x and PM emissions from combustion

engines. The field of application is primarily in internal combustion engines for motor

vehicles, but this invention can also be utilized in other energy conversion "engines" which

utilize combustion of chemical fuels, including electric power generation plants.

The Prior Art

[0002] The growing use of motor vehicles greatly adds to the atmospheric presence of

pollutants, such as oxides of nitrogen and particulate matter, and has created a demand for a

significant reduction in such emissions.

[0003] Prior art gasoline engines generally operate with charge-air throttling and intake port

fuel injection to provide a mixture of fuel and charge-air for inducting into the combustion chambers. The term "charge-air" as used herein means either air or a mixture of air and

recirculated exhaust gas. Charge-air throttling is used to control the load (or torque) output of

the engine and results in significant efficiency penalties, especially at lower loads. Port fuel injection is used to provide good control and mixing of the required fuel with charge-air. Pre-

mixed fuel and charge-air will auto-ignite during compression, depending on the fuel and charge-air characteristics, at a certain compression ratio which corresponds to the auto- ignition temperature of the fuel and charge-air mixture. Prior art gasoline engines are

generally limited to compression ratios of between 9:1 and 10:1 to avoid uncontrolled auto-

ignition. The ignition process is initiated by the sparking of a spark-plug so that rapid

combustion begins at or near piston top dead center TDC, (generally between TDC and 20

crank angle degrees after TDC), and combustion propagation proceeds from the ignition

location as a "flame-front" traveling through the combustible mixture. At higher

compression ratios and some operating conditions, the fuel and charge-air mixture auto-

ignites in an uncontrolled manner and exhibits unacceptable "knock." Not being able to

safely operate at higher compression ratios significantly reduces the engine's efficiency

potential.

[0004] Some gasoline engines utilize direct fuel injection (fuel is injected directly into the

combustion chamber), with or without charge-air throttling. Generally, these engines operate

at low loads through stratified combustion. The fuel is injected relatively late in the

compression stroke with little or no charge-air throttling. A spark then initiates combustion

that can occur as long as the stratified mixture is within the flamability limits of the fuel.

Since the late injection allows less time for good fuel and charge-air mixing than pre-mix

operation, such combustion is characterized by higher unburned fuel and particulate

emissions. Also, localized temperatures are high and NO is formed and becomes part of the

exhaust emissions. At higher loads the beginning of the fuel injection occurs earlier to allow

more time for fuel and charge-air mixing. The earlier fuel injection limits the compression

ratio for gasoline to levels comparable to the pre-mixed engines since in effect they become

pre-mixed engines at high loads. Pre-mixed gasoline engines also experience high

combustion temperatures and produce significant NO emissions. [0005] Prior art diesel engines operate over all loads with late direct fuel injection and with

little or no charge-air throttling. Diesel engines also operate at a relatively high compression ratio (generally between 15 and 20 to one) as compared to prior art gasoline engines because

they make use of the auto-ignition properties of diesel fuel. Diesel fuel will under all

intended operating conditions auto-ignite when injected into the compressed charge-air at or

near TDC. As a result of these operating characteristics, diesel engines exhibit high

efficiency. The primary problems with diesel engines are the unburned fuel, particulate and

NO_x emissions, as previously explained for late direct injection of gasoline. Gasoline could

be used in prior art diesel engines by adding an assured source of ignition (e.g., a spark or

glow plug), but still with the same emission problems.

SUMMARY OF THE INVENTION

[0006] Accordingly, it is an object of the present invention to provide for both efficient

engine operation and extremely low levels of NO_x emissions.

[0007] It is another object of the present invention to provide a method of engine operation

wherein compression temperature and peak combustion temperature are controlled over an

engine's speed and load range.

[0008] Accordingly, the present invention provides a method of operation of an internal

combustion engine including intaking ambient air through at least one compressor to provide a charge-air at a boosted pressure and introducing the boosted charge-air to the internal

combustion engine. The method further includes introducing fuel into the internal combustion engine for combustion in admixture with the charge-air at a combustion temperature approximating a target value, producing an exhaust gas. Engine operating

conditions, inclusive of torque demand, are sensed and the boosted pressure is changed

proportional to a change in the sensed torque demand so as to maintain the combustion

temperature at approximately the target value, which value is below 2100°K.

[0009] In a preferred embodiment the method further involves passing a portion of the

charge-air through a heat exchanger with bypass of the heat exchanger by a second portion of

the charge-air. The temperature of the charge-air is sensed downstream of the bypass line and

heat exchanger and the amount of the second portion bypassing the heat exchanger is

controlled by operation of a control valve in the bypass line to bring the sensed intake

temperature to a target temperature determined in accordance with the sensed engine

operating conditions.

[0010] A portion of the exhaust gas may be recirculated for admixture with the charge-air and

fuel. In this case, oxygen concentration in the admixture is sensed and the amount of EGR is

regulated to bring the sensed oxygen concentration to a target oxygen concentration

determined for the sensed engine operating conditions. The fuel feed is controlled responsive to the sensed temperature of the charge-air intake and the sensed boost pressure.

[0011] The fuel maybe introduced into the charge-air downstream or upstream of the

compressor so that the fuel is contained within the charge-air. [0012] Thus, the present invention provides, in an internal combustion engine, high

compression ratio (e.g., generally greater than 15 to 1 in the compression stroke) operation with little or no charge-air throttling, as is characteristic of the high-efficiency diesel-cycle

engine, but without the emission problems of prior art engines, through a unique, new method

of operation.

[0013] The formation rate of the pollutant NO during the fuel combustion process can be

generally expressed in simplified form as follows:

NO formation rate = [N₂] ° [O₂]°³ exp°^4T (I)

Where: C_l5 C₂, C₃, and C₄ (C_x) are constants, [N₂] is the concentration of nitrogen, [O₂] is the

concentration of oxygen, exp is a constant, and T is the absolute temperature of the mixture.

[0014] Since temperature is an exponent in the above formula (I), it can be expected that for

given concentrations of nitrogen and oxygen, the NO formation rate increases exponentially

with temperature. This relationship is widely recognized and is shown graphically in Fig. 1

for typical engine operating conditions. Engine combustion times general fall within one to

five milliseconds. It can clearly be seen that the formation of NO will be minimal if the

engine combustion temperature can be maintained below about 2000 degrees Kelvin(K). The

desirability of maintaining combustion temperatures below this level and yet still have combustion rapid enough to be complete for practical engine speeds, is well known. A recent

Society of Automotive Engineers Technical Paper (#2000-01-1177) by Patrick F. Flynn and others from Cummins Engine Company reflects prior art understanding of the known

methods of operation of engines with the goal of controlling combustion temperature to

reduce formation of NO. This paper concludes that, for gasoline and other spark-ignition

engines, "the minimum possible peak combustion temperature is 2100K" and "NO_x numbers

show a limit of 0.5g bhp-hr," and that, for diesel engines, "the lowest possible peak

combustion temperature would be approximately 2300K" with a "NO_x emissions level of

l.Og/bhp-hr."

[0015] The present invention provides a new method of engine operation that yields stable

and efficient combustion at temperatures below 2100K. Results for NO_x emissions with

gasoline, diesel and other fuels are consistently less than 0.2g/bhp-hr, substantially below that

for prior art engines.

[0016] Referring again to the Equation I for NO formation rate, the oxygen concentration

must be sufficient to fully react with the available fuel for a given engine operating condition,

e.g., charge-air boost level. The nitrogen concentration is naturally high in the charge-air and,

therefore, temperature is the one variable available for control to limit NO formation, once

the oxygen concentration is minimized and optimized for the operating condition. It is also

critical to control local temperature as NO is rapidly formed wherever the temperature is

above 2000K.

[0017] In control of the peak temperature of combustion by the method of the present invention two factors are most important. First, the temperature, T_l5 of the charge-air, or the charge-air fuel mixture if fuel is pre-mixed, at the beginning of compression, must be

controlled. Generally, the objective is to minimize T_l5 since the compression process is a

multiplier of T_j (in degrees absolute). For an ideal gas, the final compression temperature, T₂,

is a function of the compression ratio, CR, assuming adiabatic compression, i.e., T₂ ⁼ T₁ f(CR)

(where f(CR) is a function of CR). For example, for a compression ratio of 16, the multiplier

of T_t is about 3. Therefore, if the T were 300° Kelvin (27° Celsius), T₂ would be 900K.

However, if T^ were 400° Kelvin (127° Celsius), T₂ would be 1200K.

[0018] Second, for a given quantity of fuel to be burned, assuming adiabatic combustion, the

final combustion temperature, T₃ can be calculated as follows: T₃=T₂ + H_c/C_v (where H_c=heat released from combustion of the fuel and C_v is the total heat capacity of the charge-air fuel

mixture, i.e., the mass of the mixture times the specific heat capacity). Since for a given

quantity of fuel burned, H_c is fixed, the only variable available for control of T₃ is C_v. If C_v is

large, T₃ will be lower. The quantity of fuel burned, in turn, is a function of (proportional

to) torque demand.

[0019] The present invention controls T₃, for example to 2000° Kelvin (see Fig. 1), to

minimize NO formation. Therefore, for a constant T₂, H_c/C_v must be maintained constant.

To maintain H_c/C_v constant, C_v must increase as the quantity of fuel combusted (engine load)

is increased. Since C_v is of the form C^C_yM (where c_v is the specific heat capacity of the

charge-air fuel mixture and M is the mass of the charge-air fuel mixture), M must be

increased as the quantity of fuel combusted is increased and decreased as the quantity of fuel

combusted is decreased. This is accomplished in the present invention by controlling the

boost pressure of the charge-air in the intake system, i.e., controlling the charge-air density. The mass M is proportional to the pressure of the charge-air.

[0020] Another important factor to consider and account for is the fact that a real engine is

not adiabatic. If the temperature of the charge-air as it enters the intake system is T₀, and is at

a temperature lower than portions of the intake system, heat will flow from the intake system

into the charge-air and its temperature will rise. Also, as the charge-air is inducted past the

intake valve(s) into the engine cylinder, it will be exposed to the hot surfaces of the cylinder

head, piston top and cylinder walls, surfaces that were just prior exposed to the combustion

process and the hot (e.g., 2000°K at the end of combustion) combustion gases during the

period of expansion and exhaust strokes (for a four-stroke engine). Therefore, heat will flow

into the charge-air during the intake stroke and early portions of the compression stroke. To

control (usually minimize) the rise in temperature of the charge-air prior to the compression

temperature rise, the method of operation of the present invention primarily utilizes control of the boost pressure. As the boost pressure controls the mass of charge-air (or if fuel is present,

the charge-air/fuel mixture, both also referred to herein as "charge mass" or simply "charge"),

it directly controls the temperature rise of the charge mass since, for a given heat energy flow

into the charge mass from the system surfaces, the temperature rise is directly proportional to

the mass of the charge, as shown by the relationship: T^T_Q+T C,,, where H_w is the heat

energy from the system surfaces. C_v is increased to reduce T_j by increasing charge mass as

previously shown.

[0021] The method of operation of the first and second embodiments of the present invention

controls T₂ to assure that auto-ignition and combustion are sufficiently rapid to be efficient

and effective over the operating speed and load of practical engines (generally combustion that is 90% complete within 1 to 5 milliseconds), and controls T₃ to assure minimal NO

formation. Controlling T₃ to the levels of the present invention also improves engine

efficiency since the lower than prior art T₃ levels reduce the heat loss from the combustion

gases during expansion. Heat (energy) that flows through the system walls to the engine

"coolant" would, if retained in the combustion gases, be used to sustain higher system

pressures during expansion and thus extract more useful work for a given quantity of fuel

burned. With a lower T₃ the temperature difference, ΔT, between the combustion gases and

the system surfaces is lower and less heat energy flows to the coolant.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Fig. 1 is a graph of NO emission levels versus engine combustion temperature in

degrees Kelvin;

[0023] Fig. 2 is a schematic diagram of an engine drivetrain operated in accordance with the

method of the present invention; and

[0024] Fig. 3 is a schematic diagram of an engine cooling system which may be utilized in

the method of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

First Embodiment

[0025] Fig. 2 illustrates a first embodiment of the invention in a controlled temperature

combustion engine 22 operating with a fuel that is characterized by relatively low octane and relatively high cetane, i.e., a fuel with a relatively low auto-ignition temperature like

conventional diesel fuel. In the first embodiment (and in all embodiments) the final

combustion temperature (T₃) is controlled at or below about 2000°K in order to minimize

NO_x formation. T₃ is controlled by (1) control of boost pressure, (2) control of the final

compression temperature (T₂), each of which is described in more detail below. The first

embodiment operates with a compression ratio conventional for diesel engines, i.e., within a

range of about 16:1 to 20:1. Auto-ignition occurs upon injection of fuel. Accordingly,

location of auto-ignition is controlled by control of timing of fuel injection, preferably from about 20° BTDC to near TDC to produce peak cylinder pressure from immediately after TDC

to near 15° after TDC.

[0026] To enable the utilization of a high compression ratio (e.g., greater than 15) for high

efficiency, the engine relies on the primary fuel supply through direct cylinder fuel injectors

23. Intake air enters at port 11 and its flow may be throttled by optional valve 12. Exhaust

gas is mixed with intake air at port 13 forming the charge-air mixture. Exhaust gas is routed

from the exhaust pipe at port 16 through exhaust gas cooler 17, with optional condensate

return to exhaust line 18, through optional exhaust gas flow control valve 14, to port 13. The

primary exhaust gas recirculation (EGR) control valve 12' is located just downstream of port

16 in the exhaust pipe. By restricting flow through valve 12', the exhaust gas flow rate to port

13 is controlled.

[0027] The method of the present invention may utilize maps, established by operation of a

particular engine in the manner previously described to achieve a target NO emission level

and stored in the controller 26, to specify the optimum boost level and charge-air temperature (together resulting in a desired charge-air mass flow rate), optimum intake charge-air and/or exhaust oxygen concentration(s), and desired fuel rate for each speed and load (torque) over

which the engine is specified to operate, to maintain the combustion temperature at the target

level below 2100°K, e.g., 2000°K.

Control of Boost Pressure

[0028] Charge-air flows through and is compressed by compressor 19. Compressor 19 may

be a single stage compressor or two or more compressors in parallel or in series and is

primarily driven by exhaust gas expander motor 27 to provide a controlled boost pressure

level to intake manifold 21. Boost pressure level is determined by boost charge-air pressure

sensor 31. Controller 26 sends appropriate signals to expander motor 27 to control boost. An

optional electric or hydraulic motor 28 may be used and controlled by controller 26 to provide

rapid boost level changes to assist the exhaust expander motor 27 in providing rapid torque

response, since torque is dependent on boost level and boost level is controlled for controlled

temperature combustion as previously described. Controller 26 therefore sends appropriate

signals to motor 28 to control boost level during transients and during any operating

conditions where motor 27 alone cannot supply sufficient boost pressure.

Control of Charge- Air Temperature

[0029] Compressed charge-air flows through heat exchanger 20 to intake manifold 21. Heat

exchanger 20 contains a by-pass line 60 and a by-pass control valve 61 and controller 26

adjusts the control valve 61 to control charge-air temperature (T^. Charge-air temperature T_x

is determined by temperature sensor 30 for input to controller 26. Heat exchanger 20

operates as a "cooler" to cool the charge-air (the normal mode in this embodiment) or heat the charge-air as necessary to control charge-air temperature to a desired level. The heat exchange medium used in heat exchanger 20 may be the ambient air, engine coolant, exhaust

gas, etc.

[0030] Further, in accordance with the present invention, Tj may also be controlled by

controlling the temperatures of the surfaces exposed to the charge mass since heat energy

flow occurs according to the relationship: H_w = (T_S-T'₀)W, where T_s is the temperature of

exposed surfaces (wliich of course varies with location and time), T'₀ is the temperature of the

charge mass at the exposed surface, and W is an overall heat transfer coefficient. Control of

T_s is accomplished by control of engine coolant temperature, T_c, and coolant flow rate F. F

directly controls the heat transfer coefficient from the coolant to the "surface" (assuming a

small and fixed temperature difference across the system walls). T_s therefore varies according

to the relationship: T_s = B T_CF, where B is a constant. T_c may be controlled by controlling

the degree of external cooling in the coolant heat exchanger (commonly referred to as the

radiator), by adjusting the cooling surface area exposed to ambient air flow and/or by

controlling the ambient air flow through the coolant heat exchanger, usually, by varying the

speed of the "radiator" fan while F may be controlled by an electronically controlled valve in

the engine coolant supply or discharge line. T_c is controlled to a coolant supply target

temperature provided in a look-up map as a function of engine speed and load. F is also controlled to a target level provided in a look-up map as a function of engine speed and load.

Fig. 3 illustrates the cooling system for engine 22. Engine coolant flow rate F to and from

engine 22 is controlled by coolant pump 80. Engine coolant temperature T_c is controlled at

the exit of radiator 81 by varying the speed of fan 82. Control of O Concentration

[0031] Optional port fuel-injectors 53 may be used in conjunction with direct fuel injectors

23 to minimize particulate formation and to rapidly adjust fuel injection levels. Optional

oxygen sensor 25' maybe used to directly determine the oxygen concentration in the charge-

air as a faster control signal than relying on a calculated (or "expected") oxygen intake level

based on the exhaust oxygen sensor 25. The oxygen concentration signals are used in control

of EGR, i.e., valve 12'. Alternatively, charge-air oxygen concentration maybe determined

from the exhaust oxygen concentration, fuel-injected quantity, and the charge-air boost level

and temperature. Optional charge-air mass flow sensor 29 may also be used to provide faster

and more accurate engine control, i.e., in control of engine fuel feed rate which determines

engine torque output, and may be used as an indirect sensor in determination (i.e.,

calculation) of intake charge-air oxygen concentration. Charge-air enters the combustion

chamber (not shown) through conventional valves (not shown) in a conventional manner and exhaust gases leave the combustion chamber through conventional valves (not shown) and

leave engine 22 through exhaust manifold 24. Exhaust particulate trap oxidizer 54 removes

any particulate emissions and catalyst 51 oxidizes residual fuel and carbon monoxide. Engine

speed is provided to controller 26 by speed sensor 32. Torque command level is provided to

controller 26 by accelerator pedal sensor 33.

[0032] For a low auto-ignition temperature fuel like conventional diesel fuel, exhaust gas

recirculation (EGR) is used primarily to disperse the combustion heat-release (i.e., insure fuel

molecules uniformly mix with the charge-air to access oxygen molecules and thus spread the combustion heat release uniformly through the charge-air mass). Since the heat capacity of

EGR is similar to air, its use with such fuel is primarily to avoid localized temperatures above the NO formation threshold (e.g., 2000° K). Thus, the inlet charge-air oxygen concentration

must be controlled (considering boost level and inlet charge-air temperature, i.e., mass

charge-air flow rate) to insure that the fuel molecules will disperse throughout the charge-air

mass to access oxygen molecules for combustion. Valve 12' operates to regulate EGR

responsive to these sensed oxygen concentrations.

[0033] However, allowing some excess oxygen for a given fuel level will minimize

particulate formation due to poor combustion. For a particular operating condition, a

particular exhaust oxygen concentration will also result. For example, for an engine targeting

less than 0.2 grams/brake horsepower NO_x emissions, concentration values of intake charge-

air oxygen less than twelve to fourteen percent, and exhaust oxygen concentrations of less

than six percent work well for a fuel similar to conventional diesel fuel. For a given fuel,

there is an oxygen concentration level in the charge-air (e.g., about twelve percent for

conventional diesel fuel) that corresponds to sufficient charge-air mass to constrain (by

absorbing heat energy) the temperature rise associated with the combustion heat release,

when utilizing the available oxygen mass to reduce the oxygen concentration to a specified

target level (e.g., about six percent for conventional diesel fuel), to a peak combustion

temperature below the NO formation threshold. For this example, about 50% of the charge-

air oxygen mass was utilized in the combustion reaction. With such constraints, the only way

to increase torque output (i.e., combust more fuel) is to increase the charge-air mass as

previously described.

[0034] Therefore, for all fuel molecules to "find" sufficient oxygen molecules within the very

short time available (e.g., generally less than 5 milliseconds), the engine combustion system (combustion chamber, charge-air motion and fuel injector) must be optimally configured.

Fuel and oxygen molecules must "get together" while the system temperature is still above the rapid auto-ignition temperature of the fuel for good combustion to occur. Fortunately, the

rapid auto-ignition temperature for most fuels of this embodiment are significantly lower

(e.g., less than 1000° K) than the temperature threshold for NO formation (i.e., 2000° K).

Auto-ignition occurs rapidly at the controlled operating temperature, but the combustion rate

is determined by the rate that fuel molecules mix with the charge-air and access oxygen

molecules.

[0035] For open loop operation, controller 26 reads torque command from pedal sensor 33

and actual engine speed from speed sensor 32. For an increased torque command, controller

26 commands compressor motor 27 and, if needed, compressor motor 28 to increase boost

pressure level to the new target from stored maps associated with the commanded torque at

the measured engine speed. Controller 26 commands heat exchanger 20 by-pass control

valve 61 position from stored maps to target the desired intake charge-air temperature for the

engine operating condition. Controller 26 commands EGR valve 12' to the position from

stored maps appropriate to achieve the desired intake charge-air and exhaust oxygen concentrations. The controller 26 reads actual boost level from sensor 31 and actual intake

charge-air temperature from sensor 30 and from stored maps commands the appropriate fuel

rate.

[0036] For more accurate engine control, closed-loop control loops may be utilized. Exhaust oxygen concentration may be read from sensor 25 and, optionally, intake charge-air oxygen

concentration maybe read from sensor 25', with controller 26 comparing actual oxygen concentration to the desired level for the actual operating point (from stored maps) and

commanding the EGR valve to adjust to achieve the target oxygen concentration. Actual

boost level from sensor 31 may be compared by controller 26 to the desired level from stored

maps and motors 27 and 28 adjusted as appropriate to achieve the target boost level. In like

manner, actual intake charge-air temperature from sensor 30 may be compared by controller

26 to the desired temperature from stored maps and cooler 20 by-pass control valve 61

adjusted as appropriate to achieve the target charge-air temperature. Fuel flow rate may also

be adjusted based on the actual readings to achieve the target fuel rate.

Second Embodiment

[0037] Again with reference to Fig. 2, a second embodiment of the invention will now be

described, h this second embodiment a controlled temperature combustion engine 22

operates at diesel-like efficiency with a fuel that is characterized by relatively high octane and

relatively low cetane, i.e., a fuel with a relatively high auto-ignition temperature like conventional gasoline fuel. Although operated with conventional gasoline fuels, this

embodiment, unlike conventional gasoline engines, uses a high compression ratio, generally

in the range of 12 to 19.5:1, preferably at least 14:1 and more preferably at least 16:1. This

embodiment is further characterized by mixing fuel and charge-air before auto-ignition

(generally with mixing occurring before the last 30 crank angle degrees before compression

TDC), with a goal of homogeneous combustion, i.e., utilization of a pre-mixed charge of fuel

and charge-air.

[0038] This second embodiment, as in the other embodiments, utilizes a unique control

scheme by which T₃ is controlled for minimum NO_x by (1) control of boost pressure and (2) control of T₂. This second embodiment, like the first embodiment described above, operates

with auto-ignition but differs from the first embodiment in that the location of the combustion

event, i.e., timing of the auto-ignition, is controlled, not by timing of the fuel injection, but by

control of (in addition to boost pressure and T₂) intake charge-air oxygen concentration to

individually control the location of peak cylinder pressure for each cylinder. This second

embodiment also differs from the first embodiment in that it has more precise control of

oxygen concentration and of T₂.

Control of Charge- Air Oxygen Concentration

[0039] Because of the pre-mixed nature of the fuel and charge-air before ignition, utilizing

EGR to reduce the intake charge-air oxygen concentration is not necessary to disperse the

heat release of combustion throughout the mixture as in the first embodiment, since the fuel

and charge-air are already well mixed. However, EGR is used with this embodiment as will

be discussed in more detail later to control the concentration of charge-air oxygen to assist in

initiating auto-ignition and to control the location of the peak combustion cylinder pressure to

within an optimum range of from immediately after TDC to near fifteen crank angle degrees

after TDC. The control of T₃ (peak combustion temperature) to limit the formation of NO is

otherwise the same as was previously described.

[0040] Since controlling combustion temperature, T₃, inherently includes controlling the final

compression temperature, T₂, also as previously described, the method of operation of this second embodiment of the present invention also provides for controlled homogeneous

charge compression ignition (HCCI). This homogeneous charge auto-ignition engine of the

present invention provides high efficiency (high compression and expansion ratios and little or no charge-air throttling as described earlier), low NO_x emissions (by controlling

combustion temperature as described earlier) and low particulate emissions (due primarily to

the ability to provide a homogeneous fuel and charge-air mixture before auto-ignition and

combustion).

[0041] Referring again to Fig. 2, with the method of operation of the present invention, fuels

like conventional gasoline can operate over a wide range of speed and load at a high

compression ratio (e.g., greater than 14 at relatively low boost levels, generally below two bar

pressure, and even greater than 19 at relatively modest boost levels, generally below three bar

pressure) to attain "diesel-like" efficiencies even while relying on the primary fuel supply

through the port fuel injectors 53. Operation according to this second embodiment is

identical to that presented for the first embodiment except for the distinctions described here.

The optional direct fuel injectors 23 may be used for certain high power modes when utilizing

higher compression ratios (e.g., greater than 18), but the likely increase in particulate

emissions and greater reliance on trap oxidizer 54 makes the utilization of these injectors

unattractive. Heat exchanger 20 is more likely to operate in the charge-air heating mode (as

previously described) than in the first embodiment, with heat additions from exhaust gas

more likely with light loads of low compression ratio engine configurations.

Control of T₂

[0042] The method of operation of the second embodiment is distinguished from the first

embodiment further by the need to more precisely control T₂ (final compression temperature)

and charge-air oxygen concentration, as just discussed, to insure auto-ignition and control the location of peak combustion cylinder pressure (or the rate of combustion cylinder pressure rise or the level of the peak combustion cylinder pressure). The initiation of auto-ignition and

rapid combustion is generally described as follows:

Combustion initiation and combustion rate =

Where: C_x are constants, [HC] is the concentration of the fuel, [O₂] is the

concentration of oxygen, exp is a constant, and T is the absolute temperature

of the mixture.

[0043] Like the formation reaction for NO previously described, fuel combustion initiation

and reaction rate are strongly influenced by the final compression temperature, T₂. In this

case, controlling the initiation and rate of combustion is also strongly influenced by the

concentration of oxygen, since at a given engine load level, the fuel concentration is fixed.

For example, the engine maps established and stored in controller 26 as previously described,

for a low compression ratio configuration of the embodiment (e.g., 14) operating at light load,

may choose to utilize a lower T₂ with a higher oxygen concentration to minimize system cost.

At higher compression ratios (e.g., 18) and high load, controlling the initiation of auto-

ignition and the location of the peak cylinder combustion pressure, requires maps which strive to minimize T₂, since reducing oxygen concentration will require higher boost levels at

a higher cost. [0044] The description of the closed loop operation as previously described for the first

embodiment also applies to this second embodiment, but one important enhancement is

replacement of direct fuel injectors 23 (Fig. 2) with sensors 23' which sense the cylinder

pressure rise associated with combustion. Sensors 23' are used in conjunction with

conventional crank angle position sensor 34 to determine the crank angle position where

auto-ignition or preferred peak cylinder combustion pressure occurs. Sensors 23' may be

direct cylinder pressure sensors or may determine cylinder pressure through indirect means,

e.g., knock sensors or load sensors. Sensors 23' provide input to controller 26 for establishing

a closed loop signal to adjust one of the previously described parameters that determine said

position to achieve the optimum position as determined by controller 26 from stored maps.

[0045] Thus, in the second embodiment location of the combustion event, i.e., location of

peak cylinder pressure, results from the final compression charge-air temperature T₂ and the

oxygen concentration in the charge-air to the intake manifold. Controller 26 determines the

optimum crank angle for the location of the combustion event, for a given load and speed of

the engine, from a map stored in memory and adjusts T₂ and/or oxygen concentration to

levels determined from maps stored in memory. The controller 26 controls oxygen

concentration in the charge-air to the intake manifold in the manner previously described in

connection with the first embodiment, i.e., by operation of valves 14 and 12' and by control of

the boost level. Controller 26 may, in the alternative or additionally, confrol T₂ through

control of T_} by adjustment of the. bypass control valve 61 in the bypass line 60 around heat

exchanger 20. Signals from sensors 23' enable closed loop control of the location of the

combustion event by input to the confroller 26 of a signal representing the actual, detected

crank angle location of the combustion event. Responsive to said signal, the controller 26 adjusts T₂ and/or oxygen concentration in the charge-air in order to bring the actual crank angle location at which peak combustion pressure occurs to coincide with the target value for

crank angle location at which the peak combustion cylinder pressure should occur, by means

of adjustment of T_t (and T₂) and/or by means of adjustment of oxygen concentration in the charge-air as previously described.

[0046] As in the first embodiment T_j may also be controlled by controlling the temperatures

of engine surfaces contacting the charge-air in the manner previously described.

[0047] Of course, T₂ is also dependent upon T₃ and the second embodiment may be combined

with confrol of T₃ as described for the first embodiment.

[0048] This embodiment could also be operated with controlled auto-ignition at light loads

(e.g., less than 30% of maximum load) and control of oxygen concentration to stoichiometric

for higher loads to allow for effectiveness of a three-way, NO reducing catalyst 51. This

engine configuration would require greater attention to controlling T_x to control T₂, but the

reduced boost levels allowed (but higher T₃ and NO formation) with exhaust catalytic

reduction of NO, would reduce the cost of the engine.

Third Embodiment

[0049] A third embodiment of the invention also provides diesel-like efficiency, but in an

engine which utilizes a pre-mixed charge of very high octane fuel (research octane rating >90

or more, preferably > 100) such as methanol or ethanol and certain gasolines, with spark or similar controlled ignition means. The method of operation of the present invention is utilized to control the compression temperature T₂ and oxygen concentration in the charge-air

to avoid auto-ignition, and thus allows the ignition source to ignite the charge-air fuel mixture

at an optimum time to achieve high efficiency in a high compression engine. The

compression ratio will preferably be 15:1 to 20:1 and more preferably about 19:1 or more. In

the case of such engines, all engine features are as previously described with reference to Fig.

2, except that item 23 becomes spark plug 23". As in the previously described embodiments

T₂ is controlled by control of T_t and boost pressure.

[0050] Spark timing in the third embodiment is preferably about 30° BTDC to near TDC to

produce peak cylinder pressure from immediately after TDC to about 15° after TDC.

[0051] This third embodiment, like the first and second embodiments, controls the location of

peak cylinder pressure, individually for each cylinder, to within a range of from immediately

after TDC to near fifteen degrees after TDC.

[0052] Other embodiments which operate with controlled T₂ at a level just prior to auto-

ignition and which utilize a controlled igmtion source (e.g., spark plug or glow plug) to

initiate combustion, and embodiments which utilize combination operating strategies of

controlled auto-ignition and assisted ignition (where the direct fuel injectors 23 of Fig. 2

would be replaced with the ignition source) should be obvious to one skilled in the related art.

[0053] The invention maybe embodied in other specific forms without departing from the

spirit or essential characteristics thereof. The present embodiments are therefore to be

considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes

which come within the meaning and range of equivalency of the claims are therefore intended

to be embraced therein.

Claims

I Claim:

1. A method of operating an internal combustion engine comprising:

intaking ambient air through at least one compressor to provide a charge-air at a

boosted pressure and introducing the charge-air to the internal combustion engine;

introducing fuel into the internal combustion engine for combustion in admixture with

the charge-air at a combustion temperature approximating a target combustion temperature,

and producing an exhaust gas;

sensing engine operating conditions inclusive of torque demand; and

changing the boosted pressure proportional to a change in the sensed torque demand

so as to maintain the combustion temperature at approximately the target combustion

temperature, said target combustion temperature being below 2100°K.

2. The method of claim 1 further comprising:

passing a first portion of the charge-air through a heat exchanger for heat exchange

and by-passing a second portion of the charge-air through a by-pass line around said heat

exchanger;

sensing intake temperature of the charge-air downstream of the by-pass line and the

heat exchanger;

controlling the amount of said second portion by-passing the heat exchanger by

operation of a control valve in the by-pass line to bring the sensed intake temperature to approximately a target charge-air temperature.

3. The method of claim 2 further comprising:

recirculating a portion of the exhaust gas for admixture with said charge-air and fuel;

determining oxygen concentration in the admixture; and regulating the portion of exhaust gas recirculated to bring the determined oxygen

concentration to a target oxygen concentration determined for the engine operating

conditions.

4. The method of claim 3 further comprising:

sensing the boosted pressure;

controlling rate of fuel infroduction into the internal combustion engine responsive to

the sensed charge-air intake temperature and the sensed boosted pressure.

5. The method of claim 1 further comprising: recirculating a portion of the exhaust gas for admixture with said charge-air and fuel;

sensing oxygen concentration in the admixture; and

regulating the portion of exhaust gas recirculated to bring the sensed oxygen

conditions.

6. The method of claim 1 further comprising:

sensing the boosted pressure; controlling rate of fuel infroduction into the internal combustion engine responsive to

the sensed charge-air intake temperature and the sensed boosted pressure.

7. The method of claim 1 wherein said fuel is introduced into the ambient air either upsfream

or downstream of the at least one compressor so that the fuel is contained within the charge-

air.

8. The method of claim 1 further comprising: circulating engine coolant at a flow rate F and in a heat exchange relationship with

engine surfaces contacting the charge-air; and

confrolling the flow rate F responsive to changes in the engine speed and load.

9. The method of claim 8 further comprising:

circulating the engine coolant through a radiator; and

controlling cooling of the engine coolant in passage through the radiator responsive to

changes in the engine speed and load.

10. The method of claim 1 further comprising:

circulating engine coolant in a heat exchange relationship with engine surfaces

contacting the charge-air and through a radiator; and

controlling cooling of the engine coolant in passage through the radiator

responsive to changes in the engine speed and load.

11. The method of claim 1 wherein the fuel is diesel fuel and wherein auto-ignition occurs

when the fuel is introduced into a cylinder of the internal combustion engine.

12. The method of claim 1 wherein the internal combustion engine is operated with a

compression ratio of at least 12:1 and wherein the fuel is gasoline, and further comprising:

recirculating a portion of the exhaust gas for admixture with the charge-air and fuel;

determining peak cylinder pressure;

determining oxygen concentration in the admixture; and

regulating the portion of exhaust gas recirculated to produce auto-ignition and the

peak cylinder pressure within a predetermined location in a combustion cycle.

13. The method of claim 12 wherein said predetermined location in the combustion cycle is

within a range of from immediately after TDC to near fifteen degrees after TDC.

14. The method of claim 3 wherein the compression ratio is 15:1 to 20:1, wherein the fuel

has an octane rating of 90 or more, wherein the location of the combustion event is controlled

by spark timing and wherein auto-ignition is prevented by confrol of boost pressure, oxygen

concentration in the charge-air and charge-air temperature.

15. A method of operating an internal combustion engine comprising:

boosted pressure and introducing the boosted charge-air to the internal combustion engine;

the charge-air, producing an exhaust gas;

sensing torque demand; and changing the boosted pressure proportional to a change in the sensed torque demand

so as to maintain an approximately constant ratio Hc/c VI wherein He is the heat of combustion, c_vis the specific heat capacity of the admixture of charge-air, and M is the mass

of the charge-air.

16. A method of operating an internal combustion engine comprising: intaking ambient air to provide a charge-air and introducing the charge-air to the

internal combustion engine;

and producing an exhaust gas;

exchanger;

heat exchanger;

determining oxygen concentration in the admixture;

sensing engine operating conditions inclusive of torque demand, crank angle and

combustion pressure; and

determining a target crank angle for peak combustion pressure at the sensed torque

demand;

bringing the sensed crank angle into conformance with the target crank angle by

controlling the amount of said second portion by-passing the heat exchanger by operation of a

control valve in the by-pass line to adjust the sensed intake temperature and/or by regulating

the portion of exhaust recirculated.

17. The method of claim 16 further comprising:

compressing the charge-air to provide charge-air at a boosted pressure; sensing the boosted pressure; and

controlling rate of fuel introduction into the internal combustion engine responsive to

the sensed charge-air intake temperature and the sensed boosted pressure.

18. The method of claim 16 further comprising:

exchanger; sensing intake temperature of the charge-air downstream of the by-pass line and the

heat exchanger;

confrolling the amount of said second portion by-passing the heat exchanger by

operation of a control valve in the by-pass line to bring the sensed intake temperature to

approximately a target charge-air temperature.

19. The method of claim 16 further comprising:

circulating engine coolant in a heat exchanger relationship with engine surfaces

contacting the charge-air and through a radiator; and

controlling cooling of the engine coolant in passage through the radiator

responsive to changes in the engine speed and load.

20. The method of claim 16 further comprising:

recirculating a portion of the exhaust gas for admixture with said charge-air and fuel.

21. A method of operating an internal combustion engine comprising:

introducing a gasoline fuel and charge-air into the internal combustion engine for

combustion with a compression ratio of at least 12: 1 , at a combustion temperature

approximating a target combustion temperature, and producing an exhaust gas; recirculating a portion of the exhaust gas for admixture with the charge-air and fuel;

determining peak cylinder pressure;

determining oxygen concentration in the admixture; and

peak cylinder pressure within a predetermined location in a combustion cycle.

22. The method of claim 21 further comprising:

exchanger;

heat exchanger; confrolling the amount of said second portion by-passing the heat exchanger by

approximately a target charge-air temperature.

23. The method of claim 22 further comprising:

circulating engine coolant in a heat exchange relationship with engine surfaces

contacting the charge-air and through a radiator; and

confrolling cooling of the engine coolant in passage through the radiator to assist in regulating the auto-ignition and peak cylinder pressure to within said predetermined location.

24. The method of claim 23 wherein said predetermined location in the combustion cycle is

within a range of from immediately after TDC to near fifteen degrees after TDC.

25. The method of claim 21 wherein said predetermined location in the combustion cycle is

within a range of from immediately after TDC to near fifteen degrees after TDC.

26. A method of operating an internal combustion engine comprising:

introducing a fuel having an octane rating of at least 90 and charge-air into the internal

combustion engine for combustion in admixture with a compression ratio of 15:1 to 20:1, at a

target combustion temperature predetermined to minimize NO_x formation, and producing an

exhaust gas;

determining peak cylinder pressure;

determining oxygen concentration in the admixture;

igniting the admixture, at a predetermined timing, with a spark from a spark device;

and

regulating that portion of exhaust gas recirculated to regulate the final compression

temperature and to avoid auto-ignition.

27. The method of claim 26 further comprising:

passing a first portion of the charge-air through a heat exchanger for heat exchange and by-passing a second portion of the charge-air through a by-pass line around said heat exchanger;

sensing intake temperature of the charge-air downsfream of the by-pass line and the heat exchanger;

controlling the amount of said second portion by-passing the heat exchanger by

approximately a target charge-air temperature.

28. The method of claim 27 further comprising:

circulating engine coolant in a heat exchange relationship with engine surfaces

contacting the charge-air and through a radiator; and

controlling cooling of the engine coolant to assist in control of the final compression

temperature.